1. Field of the Invention
The present invention relates to an image processing apparatus and method, and more particularly, to an apparatus and method for controlling contrast of an image signal.
2. Description of the Related Art
A variety of flat display panels, such as liquid crystal displays and plasma display panels, are widely used with recently developed electronic technology. In these flat display panels, a driving circuit applies image signals to data lines, while it sequentially drives multiple scan lines of the flat display panel to display a predetermined image. The driving circuit controls contrast of the image signals to optimize the image displayed on the flat display panel.
Examples of methods of controlling contrast of image signals include a look-up table method, a histogram sliding method and a contrast stretching method.
The look-up table method uses image signals corresponding to respective pixels as indices of arrangement, stores output image signals corresponding to input image signals in a look-up table, and outputs output signals corresponding to predetermined input image signals, thereby controlling contrast. The look-up table method requires a process of calculating an output image for each image signal in order to make a look-up table in advance, and requires a memory for storing the look-up table. Also, since the look-up table method controls contrast for a next image signal using a previous image signal, the look-up table method is difficult to apply to an image signal having a variety of changes.
The histogram sliding method obtains a histogram that has measured the number of pixels having a predetermined value on the basis of input image signals, and applies a predetermined weight on the input image signals to extend or reduce the histogram, thereby controlling contrast. However, because histogram distribution varies for each input image signal, there is a high possibility that underflow and overflow will occur. Therefore, a separate clamping circuit for preventing the underflow and overflow is required.
The contrast stretching method is suitable for projection of intermediate brightness. The contrast stretching method measures the number of pixels having the same value from input image signals to obtain a histogram, and subtracts a lowest pixel value from a current pixel value to determine weight. However, because the contrast stretching method includes a multiplier and an accumulator having a complicated structure, the contrast stretching method is difficult to apply, particularly where image signals are displayed in real-time. Also, the contrast stretching method is not suitable for processing image signals distributed on a region having high or low contrast.